(a) Field of the Invention
The present invention relates to a display device, and more specifically, to an organic electroluminescence (EL) display using electroluminescence of an organic material.
(b) Description of the Related Art
An organic EL display is a display device that electrically excites a fluorescent organic compound to emit light. The organic EL display drives N×M organic light-emitting cells arranged in a matrix form with voltage or current to display images. An organic light-emitting cell of the organic EL display has a diode characteristic, so it can also be referred to as an organic light-emitting diode (OLED). Referring to FIG. 1, the organic light-emitting cell includes an anode (ITO), a multi-level organic thin film, and a cathode (metal). The multi-level organic thin film includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) to improve light emission efficiency. In addition, the organic thin film further includes an electron injecting layer (EIL) and a hole injecting layer (HIL). M×N organic light-emitting cells each of which has the aforementioned structure are arranged in a matrix form to construct an organic EL display panel.
A method of driving the organic EL display panel can be classified as either a passive matrix method or an active matrix method using thin film transistors (TFTs). The passive matrix method forms anodes and cathodes in an intersecting manner and selects a line to drive the organic EL display panel. The active matrix method couples a TFT to each ITO pixel electrode and drives the organic EL display panel based on a voltage sustained by capacitance coupled to the gate of the TFT.
FIG. 2 shows a conventional active matrix organic EL display using TFTs. Referring to FIG. 2, the organic EL display includes an organic EL display panel 100, a scan driver 200, and a data driver 300. The organic EL display panel 100 includes a plurality of data lines D1 through Dm arranged in a row direction, a plurality of scan lines S1 through Sn arranged in a column direction, and a plurality of pixel circuits 110. The data lines D1 through Dm transmit a data signal representing a video signal to the pixel circuits 110, and the scan lines S1 through Sn transmit a select signal to the pixel circuits 110. Each pixel circuit 110 is formed at a pixel region defined by two data lines and two scan lines adjacent to the pixel region.
The scan driver 200 sequentially applies the select signal to the scan lines S1 through Sn and the data driver 300 supplies a data voltage corresponding to a video signal to the data lines D1 through Dm.
The scan driver 200 and/or data driver 300 can be coupled to the display panel 100 or mounted in chip form on a tape carrier package (TCP) attached to the display panel 100 and coupled thereto. Furthermore, the scan driver 200 and/or data driver 300 can be mounted in chip form on a flexible printed circuit (FPC) or a film attached to the display panel 100 and coupled thereto. Alternatively, the scan driver 200 and/or data driver 300 can be directly mounted on a glass substrate of the display panel 100. In addition, the scan driver 200 and/or data driver 300 can be directly mounted on the glass substrate such that it can replace driving circuits formed of the same layers of the scan lines, data lines, and TFTs.
FIG. 3 is a circuit diagram of one of the M×N pixel circuits (or cells) of the display panel 100 shown in FIG. 2. Referring to FIG. 3, the pixel circuit includes an organic EL diode OLED, two transistors including a switching transistor SM and a driving transistor DM, and a capacitor Cst. The transistors SM and DM are PMOS transistors.
The driving transistor DM has a source coupled to a power supply voltage Vdd. The capacitor Cst is coupled between the gate and source of the driving transistor DM. The capacitor Cst sustains a gate-source voltage (e.g., VGS) of the driving transistor DM for a predetermined period of time. The switching transistor SM transmits a data voltage from the data line Dm to the driving transistor DM in response to a select signal from the current scan line Sn.
The organic EL diode OLED has a cathode coupled to a reference voltage Vss and emits light corresponding to a current supplied thereto through the driving transistor DM. The reference voltage Vss coupled to the cathode of the organic EL diode OLED is lower than the power supply voltage VDD such that a ground voltage can be used as the reference voltage Vss.
The active matrix pixel circuit described above must include the switching transistor SM that transmits a data signal from a data line in response to a select signal from a scan line. However, leakage current may flow through the switching transistor SM and result in erroneous operation of the pixel circuit. Therefore, a pixel circuit capable of transmitting a data signal without having current leakage is required.